The closed loop bandwidth for a conventional current feedback operational amplifier is given by: EQU B.sub.W =[C.sub.c (R.sub.2 +G R.sub.in)].sup.-1 (Eq. I)
Here, C.sub.c is the compensation capacitance at the high gain node of the operational amplifier, R.sub.2 is the feedback resistor, and G is the closed loop voltage gain for the amplifier, which is equal to (1+R.sub.2 /R.sub.1), where R.sub.1 is the gain-setting resistor between the inverting input of the operational amplifier and ground.
For an ideal current feedback operational amplifier, the input impedance, R.sub.in, is zero, and consequently, the closed loop bandwidth of the circuit is independent of the amplifier gain. In conventional current feedback operational amplifiers, however, R.sub.in is non-zero, its magnitude being determined by the intrinsic emitter resistance of the transistors which form the inverting input of the amplifier. This emitter resistance depends on the bias current in the transistor, and for a bipolar transistor it is typically on the order 26 ohms per 1 mA of emitter current.
The feedback resistor, R.sub.2, is typically on the order of a few thousand ohms, and as long as the product, G R.sub.in, is small relative to R.sub.2, the closed loop bandwidth of the amplifier will be virtually independent of gain. However, since R.sub.in is non-zero, G R.sub.in will be comparable R.sub.2 for sufficiently high gains, causing a roll off in B.sub.W.
The transfer function of a closed loop current-feedback operational amplifier and the DC closed-loop gain accuracy, A.sub.c also roll off for large gains. The transfer function is given by: EQU V.sub.out /V.sub.IN =G Z.sub.t /(Z.sub.t +R.sub.2 +G R.sub.in),(Eq. II)
where Z.sub.t is the open loop transimpedance gain. As with B.sub.W, the transfer function will begin to decrease as the product G R.sub.in becomes comparable to R.sub.2 at high gains. Similarly, the DC closed-loop gain is given by: EQU A.sub.c =G Z.sub.o /(Z.sub.o +R.sub.2 +G R.sub.in) , (Eq. III)
where Z.sub.o is the DC open loop transimpedance gain. As with B.sub.W and the transfer function, the gain accuracy falls off for large gains due to its inverse dependence on the factor G R.sub.in.
In addition to these problems, the inverse dependence of R.sub.in on the transistor bias current introduces non-linearities into the circuit response for large input signals. While the effect of such non-linearities can be partially offset by operating the amplifier at higher bias currents, this approach increases power dissipation and noise in the amplifier circuit.